1. Field of the Invention
The present invention relates to a plasma apparatus for generating plasma by irradiating electron beams introduced into a processing chamber upon a reactive gas to process a substance.
2. Description of the Prior Art
A conventional plasma apparatus will be first explained hereinbelow. When a glow discharge is generated between an anode and a cathode both arranged within a plasma generating chamber disposed in front of a processing chamber, a plasma can be generated from an argon gas introduced into the plasma generating chamber. Further, when ions in the generated plasm collide against the cathode, since the cathode is heated, thermoelectrons are generated from the cathode. The generated thermoelectrons allow the plasma into a further high density state. Within the plasma generating chamber, electrodes are further arranged to generate an electric field. The electrons in the generated plasma are further accelerated by the electric field generated between the electrodes into electron beams. The generated electron beams are introduced into a processing chamber.
The electron beams are irradiated upon a reactive gas introduced into the processing chamber, so that plasma is generated. Within the processing chamber, a semiconductor wafer is mounted on a sample base arranged in a direction perpendicular to the irradiation direction of the electron beams. Therefore, it is possible to process (e.g., etching, semiconductor film deposition, etc.) the wafer by the plasma generated within the processing chamber.
In the conventional plasma apparatus as described above, however, since the wafer is mounted in the direction perpendicular to the irradiation direction of the electron beams, the semiconductor wafer is irradiated with the electron beams roughly perpendicularly, so that there exists a problem in that ions enter deep into the wafer and thereby the wafer characteristics deteriorate.
To overcome this problem, another plasma apparatus has been proposed such that the wafer is mounted in the horizontal direction of the electron beam irradiation. In this conventional plasma apparatus by which the wafer is mounted horizontally in the electron beam irradiation direction, since the plasma generated over the wafer is not uniform in density, there arises another problem in that the wafer cannot be processed uniformly. The non-uniformity of the plasm density results from the non-uniformity of the density of the electron beams introduced into the processing chamber. In other words, the electron beams are high in density at the middle portion of the beams, but low at the outer peripheral portion thereof. It is of course possible to reduce the non-uniformity of the electron beam density by depressing the electron divergence due to collision against the reactive gas or due to the space charge effect. However, it is still difficult to process the wafer sufficiently uniform. The above-mentioned problem related to the non-uniform processing has recently become serious more and more with increasing diameter of the wafer.